Hey Christine:
I've been exploring "Medium Chill" and have been reading up on it. I hope to be prepared if I ever have to have contact with my uBPDS.
Someone with experience might have a different opinion, but I'd tend to ignore the post in all ways (no like, no comment, etc.). One of the rules of Medium Chill is NOT to share anything about yourself. Probably want to keep that in mind when you post on social media that MIL can access. Might want to think about all that you share with others that could get forwarded or shared with MIL.
What I've read about Medium Chill is:
- Tell them nothing, ask them nothing and offer vacuous pleasantries
- You are detached
- You are emotionally disassociated or indifferent
- Never share personal information about yourself
- Never get involved with their drama/problems
- When they try to pick a fight, deflect, show no interest and no response. The technique is very similar to the old advice to "play dead" with an attacking vear.
I read about a technique similar to Medium Chill, called "Robot Mode". Robot mode is supposed to be used only as a defense tactic, if you are in the presence of someone who is raging and you can't get away from them. You might want to Google "Robot Mode Narcissist". It is presented for use with Narcissists, but the skill might come in handy with a raging BPD. In a nutshell, you are using a technique to NOT react or get angry. It was suggested that you appear attentive, but you are actually showing no emotion or reaction, because you are studying them for a police lineup,

. You observe/study their hair, clothing, shoes, jewelry, etc. Focus on other things so not to show emotions.
Best wishes with Medium Chill.